Known drainage systems of the Auxiliary Power Unit (APU) of an aircraft comprise a mast for evacuating any liquid accumulated in the APU compartment to the atmosphere by gravity.
One of the effects of the ventilation of the APU compartment typically induced by a jet pump mechanism generated by the APU itself is a depression (lower pressure in the APU compartment than outside) generated in the APU compartment.
Some of the drawbacks of a pressure impact in the drainage system are mainly related to the difficulties in priming the drainage mast. That is, the draining capacity of the mast is sufficient once it is working fully filled with liquid, but if the ingestion of air into the APU compartment starts before the mast is filled with liquid, because there is a lower pressure in the APU compartment than outside, the stresses induced by the air flow impede the flow of liquid, and it may well happen that the mast never gets filled or primed. Rather, typically, air entering the APU compartment almost completely prevents any outflow of liquid until the liquid height inside the APU compartment is enough to balance the suction in the APU compartment, rendering the mast ineffective. Once this height is reached, only a reduced cross section of the mast will be useful for draining and, in addition, the liquid height in the APU compartment will be as large as what would be reached without a drainage mast.
U.S. Pat. No. 5,996,938 proposes a drainage system comprising a drain tube which is in fluid communication at its upper end with a liquid collecting floor and at its lower end to a check valve selectively open and placed as closely as possible to the aircraft overboard drain port. Liquids collected by the collecting floor are gravitationally directed into the drain tube upper end. The check valve is configured to be open to fluid drainage from the tube out of the drain port when the pressure head of fluid upstream of the check valve exceeds fluid pressure at the drain port due to aircraft operating conditions. This drainage system is therefore dependent of the said check-valve.